There are five to seven petals on rose-purple flowers that appear in columns along the upper end of stems. Bloom is in midsummer.
Purple loosestrife is a European plant probably introduced to the United States as an ornamental. It reproduces by both seed and creeping rootstocks. Infestations can impede water flow and replace beneficial plants and thus displace wildlife. It can be found in shallow marshy wetland areas and ditches.